Multifunctional tape

ABSTRACT

A method comprises forming elongate structures ( 5 ) on a first substrate ( 3 ), such that the material composition of each elongate structure ( 7 ) varies along its length so as to define first and second physically different sections in the elongate structures. First and second physically different devices ( 1,2 ) are then defined in the elongate structures. Alternatively, the first and second physically different sections may be defined in the elongate structures after they have been fabricated. The elongate structures may be encapsulated and transferred to a second substrate ( 7 ). The invention provides an improved method for the formation of a circuit structure that requires first and second physically different devices ( 1,2 ) to be provided on a common substrate. In particular, only one transfer step is necessary.

This is a divisional application of U.S. Nonprovisional application Ser.No. 12/413,613 filed on Mar. 30, 2009, which claims priority under 35U.S.C. §119(a) to British Patent Application No. 0805850.5 filed on Apr.1, 2008, the entire contents of which are hereby incorporated byreference.

FIELD OF THE INVENTION

The present invention relates to structures containing elongatestructures in which two physically different sections are defined whichsubsequently allow the formation of physically different devices, and tothe formation of such structures. It particularly relates to theformation of devices containing low dimensional structures andperforming physically different functions onto receiver substrates onwhich those devices could not be fabricated directly to achievemonolithic integration.

BACKGROUND OF THE INVENTION

Monolithic integration may be not feasible because it may be eithertechnically not possible or not cost effective.

An example where the monolithic integration may not be a cost-effectiveoption is the monolithic integration of a Complementary-Metal OxideSemiconductor (CMOS) interface on a MEMS (Micro Electro-Mechanical)sensor, which often requires running through a costly CMOS process whichwon't cover the entire substrate area.

The monolithic integration may be technically not viable because eitherthe substrate onto which the devices are to be integrated cannotwithstand the process conditions (e.g. high temperature steps), therequired material cannot be deposited with sufficient quality onto theforeign substrate (e.g. due to structural incompatibilities) or theprocess flow may be incompatible with devices previously fabricated onthe receiver substrate (e.g. high temperature steps after metallisationof previous devices or contamination issues).

Display technologies are an example where the structuralincompatibilities in conjunction with the low thermal budget of theglass substrate inhibit the formation of single-crystallinesemiconductors on amorphous glass substrate and where it is advantageousto integrate high performance semiconducting devices with differingfunctionality. Examples of such devices include npn transistors and pnptransistors (e.g. to form CMOS circuits), pressure sensors (e.g. forhaptic interfaces), light sensors (e.g. for adapting the display to theambient lighting conditions) and last but not least red, green and blueLight Emitting Devices (LEDs) (e.g. for emissive displays) on atransparent substrate such as a glass substrate or plastic substrateswhich may be flexible.

These devices may contain elongate low dimensional structures, which areformed onto a suitable substrate but can be subsequently transferredonto a different substrate. Examples of devices which may containelongate low dimensional structures are npn transistors, pnptransistors, sensors, capacitors, red, green and blue LEDs. The bulk ofthe receiver substrate may consist of glass, polymers, metals, orsemiconductors.

Where physically different structures consisting of or containing lowdimensional structures are formed on a formation substrate and aretransferred to a target substrate, it is desirable to be able toexercise a degree of control over the arrangement of these devices onthe target substrate after transfer/re-orientation, both with respect topredefined features on the target substrate and with respect to eachother.

The term “low dimensional structure” as used herein refers to astructure that has at least one dimension that is much less than atleast a second dimension.

The term “elongate structure” as used herein refers to a structurehaving at least two dimensions that are much less than a thirddimension. The definition of an “elongate structure” lies within thedefinition of a “low dimensional structure”, and a nanowire is anexample of a structure that is both a low dimensional structure and anelongate structure.

Low dimensional structures that are not elongate structures are known.For example, a ‘platelet’, which has two dimensions of comparablemagnitude to one another and a third (thickness) dimension that is muchless than the first two dimensions constitutes a “low dimensionalstructure” but is not an “elongate structure”.

For the avoidance of any doubt, the term “physically different” means inthis context that those sections of the elongate low dimensionalstructure which determine the device performance differ in at least oneof the following points:

1. material composition—e.g. doping concentrations, doping type (p and ndoped regions), semiconducting material with different band gaps;

2. material composition profile—e.g. doping profiles along the structureand/or presence of heterojunctions;

3. cross-sectional geometry—e.g. different side facets at differentsections along the low dimensional elongate structures;

4. density of elongate low-dimensional structures—e.g. the elongatelow-dimensional structures may branch into several elongatelow-dimensional structures, or some of the elongate low-dimensionalstructures may be shorter than others;

5. orientation of elongate low-dimensional structures—e.g. the elongatelow-dimensional structures may change their orientation at well definedpositions along their lengths (kinks); and

6. cross-sectional dimensions of elongate low-dimensionalstructures—e.g. different cross sectional areas of the elongate lowdimensional structures.

Additionally, the physically different sections may also differ in theirlength.

Methods are known for transferring structural features from a firstsubstrate to a second substrate. However, at present no suitabletechniques are available for applying a high density of structuralfeatures with an elongate/low dimensional geometry to a receiversubstrate such that all of the following desiderata can be met:

1. The elongate features consist of at least two differentsegments/regions, which are distinguishable from each other by differentvariations in their material composition along their longest dimension.

2. The elongate features are transferred to a target substrate with oneand only one transfer step.

3. The spatial arrangement and spacing of the elongate/low dimensionalstructures within devices defined on the target substrate can besubstantially controlled.

4. The elongate low dimensional structures are oriented on the targetsubstrate such that any arbitrary symmetric or asymmetric distributionof the material composition progresses along their longest dimension inthe same manner for all structures.

5. At least one edge of the elongate low dimensional structures isaligned with one or more common planes.

6. Physically different devices containing the same number of elongatelow dimensional structures with cross sections which scale in asubstantially similar manner can be obtained in close proximityindependent of yield and reproducibility issues related to thefabrication of the elongate low dimensional structures.

Control over one or more (and preferably all) the factors set out aboveis necessary to permit the use of such elongate or low dimensionalstructures to improve existing and develop new nanotechnologies.

U.S. Pat. No. 7,067,328 discloses a method for transferring nanowiresfrom a donor substrate (for example the substrate on which they areformed) to a receiver substrate. This is achieved by disposing anadhesion layer on the receiver substrate, and mating it with the donorsubstrate. A degree of alignment and ordering of the nanowires on thereceiver substrate is achieved by moving the donor substrate andreceiver substrate relative to one another while they are in contact.

U.S. Pat. No. 6,872,645 teaches a method of positioning and orientingelongate nanostructures on a surface by harvesting them from a firstsubstrate into a liquid solution and then flowing the solution alongfluidic channels formed between a second substrate and an elastomerstamp. The nanostructures adhere to the second substrate from thesolution with a preferred orientation corresponding to the direction offluid flow.

U.S. Pat. No. 7,091,120 discloses a process in which a liquid materialis disposed on a population of nanowires that are attached to a firstsubstrate with their longitudinal axes perpendicular to the plane of thefirst substrate. The material is then processed in order to cause it tosolidify into a matrix that is designed to adhere to the nanowires andact as a support for the nanowires during the process of separating thenanowires from a first substrate and transferring them to a secondsubstrate. Optionally, once the composite of nanowires embedded in thematrix material has been successfully transferred to the secondsubstrate the matrix material can be removed to leave only thenanowires.

U.S. Pat. No. 7,091,120 also discloses an extension to this processwhereby the composite of nanowires embedded in the matrix material islithographically patterned into blocks. The blocks are then applied to asecond substrate such that the embedded nanowires are aligned with theirlongitudinal axes parallel to the plane of the second substrate.

In one embodiment of the method of U.S. Pat. No. 7,091,120 the compositematerial is formed by unidirectionally disposing the matrix material onan ordered or random arrangement of nanowires. The directional flow ofthe matrix material induces the nanowires to orientate within thecomposite material parallel to the plane of the first substrate. Themethod of U.S. Pat. No. 7,091,120 has a number of disadvantages, asfollows:

-   -   Deposition of the matrix as a liquid may disturb the        alignment/orientation of the elongate nanostructures on the        donor substrate. Hence, it is challenging to control the        arrangement and/or orientation of the elongate structures        contained in each block relative to the external dimensions of        the block.    -   The absolute dimensions and aspect ratio of the composite blocks        are limited by the resolution, alignment accuracy and anisotropy        of the lithographic and etch processes used to pattern the        blocks (generally, only blocks with a low aspect ratio can be        obtained). Consequently, it is difficult to control the number        of elongate structures contained in each block or, again, the        arrangement of elongate structures contained in each block        relative to the external dimensions of the block.    -   The method does not easily enable nanostructures to be        reoriented from a perpendicular orientation relative to the        first substrate to a parallel orientation relative to the second        substrate.

US patent application No. 2004/0079278 discloses a method of forming acomposite material comprising an array of isolated nanowires and amatrix that fills in the gaps between the materials. This method isdesigned to fabricate monolithic photonic band gap composite structuresthat cannot easily be transferred between different substrates.

U.S. Pat. No. 7,068,898 discloses a composite structure comprisingnanostructures dispersed in a polymer matrix with random and ‘lessrandom’ orientations. The application is directed to light concentratorsand waveguides that take advantage of the anisotropic emission patternto ensure light is redirected in the guide or concentrator as desired.

Small, Vol. 1, No. 1, p. 142 (2005) describes how three LEDs werefabricated using one uniformly p-doped nanowire which is crossed bythree identical n-doped nanowires forming three pn junctions. Each pnjunction emits light at the same wavelength. This publication suggeststhe assembly of three pn junctions each emitting light at differentwavelength by replacing the three identical n-doped nanowires withnanowires consisting of three different suitable materials (GaN, CdS,and CdSe). The same approach of assembling crossing nanowires is used todemonstrate the integration of one LED with one FET, where thedifference in functionality is solely achieved by using differentoperation conditions (voltages). Science, Vol. 294 p. 1313 (2001) usesthe same assembly approach as Small, Vol. 1, No. 1, p. 142 (2005) torealise logic gates.

In both cases, the technology described requires two assembly steps tofabricate a cross-bar arrangement. Even if one could envisage a similarapproach by replacing the p-doped bottom nanowire with patterned p-dopedSi, the transfer and assembly technology described (a fluid assemblymethod) would not allow to obtain devices consisting of differentmaterial compositions in well defined spots using only one transferstep. Furthermore, it would be impossible to assemble nanowires withasymmetric doping profiles with identical orientation. Therefore, thedevice performance of each group of devices can't be optimisedindependent of each other unless several transfers are performed. Also,the method is not suited to assemble nanowires with asymmetric dopingprofiles which are often desired if the operating conditions (voltages)are asymmetric as in the case of transistors.

Proceedings of the IEEE, Vol. 93, No. 7, p. 1357 (2005) describes alogic gate realised by using a single, uniformly doped nanowire. Similarto the method described in Small, Vol. 1, No. 1, p. 142 (2005), thelogic operation requiring different devices (e.g. resistor andtransistor) is achieved by applying different operating conditions(voltages) to different segments of the nanowire. This approach suffersfrom the same drawbacks described in the previous publications as far asthe device optimisation by different and asymmetric doping profiles isconcerned.

Co-pending UK patent application No. 0620134.7 (UK patent applicationpublication No. GB2442768A) describes a method of making encapsulatedlow dimensional structures such that they are suitable to be transferredto a different substrate. During the transfer the number of elongatestructures, their alignment, spacing, and their orientation aremaintained. Furthermore, these structures can be subsequently processedinto devices using conventional lithographic methods in combination withsubtractive (e.g. dry etching) and additive techniques (e.g. metaldeposition). The number of elongate structures within each device iswell controlled. It is possible to divide each group after the transferinto smaller segments to yield multiple identical devices out of eachgroup (e.g. several npn transistors).

US2005/0180194 discloses a “nano tube cell” containing alternatingregions of p-type doping and n-type doping. This cell may beincorporated in a structure such that two PNPN diode switches aredefined in the tube cell.

US2005/0180194 does not describe in detail how the structure isfabricated. The structure shows a symmetric IV curve, which implies thatthe two diode switches are identical to one another.

US2007/0102747 relates to a carbon nano tube FET (CNTFET) structure, andproposes a structure incorporating an n-type FET and a p-type FET.However, the two FETs are physically identical, and the n-type FET andp-type FET are obtained by applying different gate voltages to the twostructures.

US2003/0089899 describes formation of regions of different doping typein a nanoscale wire to produce a single device.

SUMMARY OF THE INVENTION

The current invention addresses the challenge providing an improvedmethod of fabricating devices consisting of physically differentlow-dimensional structures on a receiver substrate such that as many aspossible, preferably all, of the following desiderata can be met:

1. The elongate low-dimensional structures are transferred to a targetsubstrate with one and only one transfer step.

2. The spatial arrangement and spacing of the elongate low dimensionalstructures within a group (defined on the formation substrate) can besubstantially controlled during the transfer.

3. The transferred elongate features consist of at least two differentsections, which are physically different from each other.

4. The transferred elongate low dimensional structures within each groupare oriented such that any arbitrary symmetric or asymmetricdistribution of the material composition progresses along their longestdimension in the same manner for all structures in a group.

5. In addition, at least one edge of the elongate low dimensionalstructures can be aligned with one or more common planes.

6. Physically different devices containing the same number of elongatelow dimensional structures with cross sections which scale in asubstantially similar manner can be obtained in close proximityindependent of yield and reproducibility issues related to thefabrication of the elongate low dimensional structures.

“Cross sections which scale in a substantially similar manner” meansthat the one-dimensional parameters describing one cross section scalesubstantially in proportion to the one-dimensional parameters describinganother cross section. Thus, two circular cross sections scalesubstantially “similar” to one another, if the radius of one crosssection scales in proportion to the radius of the other cross section(both belonging to the same elongate low-dimensional structure) for allelongate low dimensional structures. In other words, variation in theratio of the two radii measured within each elongate low-dimensionalstructure is considerably less from one elongate low-dimensionalstructure to another than variation in the absolute values of the radii.Similarly, if one cross section is circular and another cross sectionmeasured within the same elongate low dimensional structure describes ahexagon, the edge length of the hexagon divided by the radius withineach elongate low-dimensional structure varies considerably less fromstructure to structure than the absolute values of the radius and theedge length. If elongate structures are tapered (e.g. they have theshape of a truncated cone), it is important that the measurements takenalong the length of each low dimensional elongate structure are taken atequivalent positions.

The term “elongate low-dimensional structure” includes, but is notrestricted to, nanowires and nanotubes which may be obtained by additivemethods (e.g. chemical vapour deposition, molecular beam epitaxy,chemical synthesis) or subtractive methods (e.g. reactive ion etching)or any combination.

A first aspect of the present invention provides a method comprising:

-   -   forming elongate structures on a first substrate, at least one        of the material composition, the material composition profile,        the cross-section geometry, the cross-sectional dimensions and        the orientation of each elongate structure varying along its        length whereby at least first and second physically different        sections are defined in the elongate structures; and forming at        least first and second devices, the first device comprising the        first section of one or more of the elongate structures and the        second device comprising the second section of one or more of        the elongate structures. This allows the formation for two        physically different devices.

Before the fabrication of the physically different devices, the elongatestructures may be encapsulated and transferred to a second substrate.

For two sections to be “physically different” from one another, thoseregions/sections of the elongate structures which determine the deviceperformance must differ between the devices in at least one of thefollowing points: material composition; material composition profile;cross-sectional geometry; crystallographic orientation of elongatelow-dimensional structures; change in orientation (kinking) within onephysically different section (e.g. the kink is important to obtain thedesired device performance) cross-sectional dimensions of elongatelow-dimensional structures. For two devices to be “physically different”from one another, those regions/sections of the elongate structureswhich determine the device performance must differ between the devicesin at least one of the following points: material composition; materialcomposition profile; cross-sectional geometry; density of elongatelow-dimensional structures; orientation of elongate low-dimensionalstructures; cross-sectional dimensions of elongate low-dimensionalstructures. (These points are explained in more detail above.) In otherwords, the physical difference is solely defined by differences ingeometry and/or material composition and not established by for exampleimplementing a different electrostatic environment (e.g. a differentmode of operating a device). Thus, as an example, an npn transistor is a“physically different device” to a pnp transistor, since the materialcomposition and material composition profile (in particular regardingthe doping type and doping concentration) of those regions/sections ofan npn transistor which determine the device performance are differentfrom the material composition (in particular the doping type) of thoseregions/sections of a pnp transistor which determine the deviceperformance. As a further example, two light-emitting devices that havethe same general structure but that have active regions of differentmaterial composition so that the devices emit light at differentwavelengths to one another are “physically different” devices. Moreover,any change in cross section of the elongate structures, which may be thegeometry, the size or both, will give a physically different device.This is, because a change in any of these will result in a differentdevice characteristic if all other parameters remain unchanged.

It should be noted that two “physically different” sections are notrequired to be physically different along their entire length. Forexample, if it is desired to fabricate an n⁺ n⁻ n⁺ transistor as a firstdevice and an p⁺ n⁻ p⁺ transistor as a second device, the part of theelongate structure(s) that forms the n⁻ region of the n⁺ n⁻ n⁺transistor may in principle be identical to the part of the elongatestructure(s) that forms the n⁻ region of the p⁺ n⁻ p⁺ transistor.However, the overall part of the elongate structure(s) that forms the n⁺n⁻ n⁺ transistor will be physically different from the overall part ofthe elongate structure(s) that forms the p⁺ n⁻ p⁺ transistor (owing tothe different doping type required for the source and drain regionswhich also results in different material composition profiles in the twooverall parts of the elongate structure(s) that form the twotransistors).

It will also be apparent from the above that the first [second] sectionis not required to have a material composition, material compositionprofile, cross-section geometry, the cross-sectional dimensions andorientation that are uniform along the length of the first [second]section, although they may all be uniform along the length of the first[second] section. In general one or more of these properties may varyalong the length of the first [second] section so that the first[second] section may contain two or more sub-sections with one or moreproperty varying between one sub-section and another. Where this is thecase, the first section and the second section may be considered asphysically different if at least one subsection of the first sectiondiffers from the corresponding subsection of the second section in oneof the properties specified.

A second aspect of the present invention provides a method comprising:forming free-standing elongate structures on a first substrate; definingat least first and second physically different sections in the elongatestructures, at least one of the material composition, the materialcomposition profile, the cross-section geometry, the cross-sectionaldimensions or the orientation of the elongate structures being differentbetween the first region and the second region; and forming at leastfirst and second devices, the first device comprising the first sectionof one or more of the elongate structures and the second devicecomprising the second section of one or more of the elongate structures.

By “free-standing” is meant that the elongate structures are makingconnect with the first substrate only at one of their ends or only atboth of their ends (ie, they adopt an inverted “U” shape), when seenfrom the side. Free-standing elongate structures include, for example,elongate structures that are connected to the substrate only at one oftheir ends and extend generally away from the substrate.

The method of the second aspect results in the first and secondphysically different sections being defined in the elongate structuresafter the elongate structures have been fabricated, whereas the methodof the first aspect results in the first and second physically differentsections being defined in the elongate structures as the elongatestructures are grown on the first substrate. Apart from this, the methodof the second aspect is generally similar to the method of the firstaspect.

The method of the second aspect may further comprise severing aconnection between the elongate structures and the first substrate. Thismay be done either before or after the first and second physicallydifferent sections have been defined in the elongate structures.

The first and second physically different sections may be defined in theelongate structures after the elongate structures have been transferredto a second substrate (preferably with the structures having beenencapsulated before transfer to preserve their position relative to oneanother).

Alternatively, the first and second physically different sections may bedefined in the elongate structures after fabrication of the elongatestructures but while the elongate structures are still on the firstsubstrate, with the elongate structures being subsequently transferredto a second substrate if desired (preferably with the structures havingbeen encapsulated before transfer to preserve their position relative toone another). This may be done, for example, by depositing a maskingmaterial over the elongate structures, patterning the masking materialto expose selected segments of the elongate structures, and modifyingthe exposed segments of the elongate structures such that they arephysically different to the non-exposed parts of the elongatestructures.

The first and second devices are defined at different points along thelength of the elongate structures, in that at least one region whichdetermines the performance of the first device occurs at a differentpoint along the length of the elongate structures to at least one regionwhich determines the performance of the second device. This allows theperformance of the first device to be optimised substantiallyindependently of optimising the performance of the second device.(However, it is possible for the first and second devices to overlap, iefor a section of the elongate structures to be common to both the firstdevice and the second device.)

The method of the first or second aspect may comprise encapsulating theelongate structures.

The method of the first or second aspect may comprise rotating theelongate structures such that their longest dimension extends parallelto the substrate surface.

After rotating the elongate structures, the elongate structures may becoplanar with one another.

The method of the first or second aspect may comprise transferring theelongate structures to a second substrate. In the second aspect, thetransfer may be before or after definition of the first and secondsections.

The elongate structures may be formed as a group; wherein encapsulatingthe elongate structures comprises encapsulating the group of elongatestructure; and wherein defining the first and second devices comprisesdefining the first and second devices in the group of elongatestructures.

In the first aspect, the elongate structures may be formed so as to befree-standing on the first substrate.

In the first or second aspect the elongate structures may be formed soas to extend generally perpendicular to the first substrate.

The invention allows the orientation of the elongate structures to bechanged during transfer, so that the orientation of the elongatestructures relative to the second substrate (after transfer) isdifferent from the orientation of the elongate structures relative tothe first substrate (before transfer).

In the first or second aspect the elongate structures may be transferredto the second substrate so as to extend generally parallel to the secondsubstrate.

In the first or second aspect the first and second devices may bedefined such that the active region of the first device comprises thefirst section and the active region of the second device comprises thesecond section. The invention is not however limited to this, and thefirst and second sections may constitute other parts (for example acontact) of the first and second devices. As an example if the twodevices to be formed are an n+/n/n+ depletion mode transistor and ap+/n/p+ inversion mode transistor, these are physically differentdevices owing to the different material composition in the contact areaseven though the devices have identical active device regions. In thiscase, the first section may comprise an n+ contact of the depletion modetransistor and the second section may comprise a p+ contact of theinversion mode transistor.

In general, the first device may comprise one or more sections inaddition to the first section, and the second device may comprise one ormore sections in addition to the second section.

In the first or second aspect defining the first and second devices maycomprise forming at least one contact to the elongate structures.

In the first or second aspect a third sub-section of at least one of theelongate structures may be adapted for formation of a contact to thefirst device, and defining the first and second devices may compriseforming at least one contact to the third sub-section.

In the first or second aspect a fourth sub-section of at least one ofthe elongate structures may be adapted for formation of a contact to thesecond device, and defining the first and second devices may compriseforming at least one contact to the fourth sub-section.

In the first or second aspect defining the first and second devices maycomprise severing the elongate structure between the first device andthe second device.

In the first or second aspect defining the first and second devices maycomprise partially removing the encapsulant so as to expose one or moreportions of the elongate structures.

Partially removing the encapsulant may expose the third sub-section ofthe at least one elongate structure.

In the first or second aspect the method may comprise depositing anelectrically conductive material over the exposed portion(s) of theelongate structures.

In the first or second aspect defining the at least first and seconddevices may comprise defining at least first and second groups ofdevices, the devices in the first group being not physically differentfrom one another, the devices in the second group being not physicallydifferent from one another, and devices of the first group beingphysically different from devices of the second group.

In the first or second aspect encapsulating the elongate structures maycomprise encapsulating the elongate structures in a transparentmaterial.

In the first or second aspect encapsulating the elongate structures maycomprise encapsulating the elongate structures in an electricallyconductive material.

In the first or second aspect encapsulating the elongate structures maycomprise encapsulating the elongate structures in an electricallyinsulating material.

A third aspect of the invention provides a circuit structure fabricatedby a method of the first aspect or of the second aspect.

A fourth aspect of the invention provides a circuit structurecomprising:

-   -   first and second devices disposed over or fabricated on a common        substrate, each device containing elongate low-dimensional        structures;    -   wherein the first device is in close proximity to the second        device;    -   wherein at least one of the material composition, material        composition profile, cross-section geometry, cross-sectional        dimensions and orientation of the elongate low-dimensional        structures of the first device differs from the material        composition, material composition profile, cross-section        geometry, cross-sectional dimensions and crystallographic        orientation of the elongate low-dimensional structures of the        second device, whereby the first device is physically different        from the second device;    -   wherein the material composition, material composition profile,        cross-section geometry, orientation of cross-sections,        cross-sectional dimensions and orientation of the elongate        low-dimensional structures of the first device are nominally the        same for all elongate low-dimensional structures of the first        device; and    -   wherein the material composition, material composition profile,        cross-section geometry, orientation of cross-sections,        cross-sectional dimensions and orientation of the elongate        low-dimensional structures of the second device are nominally        the same for all elongate low-dimensional structures of the        second device.

If the elongate low dimensional structures of the first device have thesame orientation as the low dimensional structures of the second devicethey will also be substantially “in line” with each other. In otherwords, if the elongate low dimensional structures within the firstdevice were extended towards the second device they would overlap withthe elongate low dimensional structures of the second device. Also, ifone elongate structure within one device has a larger cross section thanthe others, this would be also true for the corresponding elongate lowdimensional structure in the other device. This allows the performanceof one device to be matched to the performance of another device,because unintended variations in geometry are present in both devices toa similar degree.

By specifying that the material composition, etc. of the elongatelow-dimensional structures of the first device are nominally the samefor all elongate low-dimensional structures of the first device is meantthat the variation in the material composition, etc. between theelongate low-dimensional structures of the first device is smallcompared to the average material composition (or intended materialcomposition), etc. of the elongate low-dimensional structures of thefirst device. As far as doping concentrations are concerned, thematerial composition and/or material composition profiles preferablydiffer by no more than an order of magnitude from the average values,more preferably differ by no more than a factor of 2 from the averagevalues or intended values, and even more preferably are within ±20% ofthe average values. All other properties are preferably within 25% ofthe nominal/intended value, and particularly preferably are within 10%of the nominal/intended value.

The reason for the wider window for doping concentrations is three-fold:

1. Doping concentrations are generally harder to control.

2. The device performance is comparably “insensitive” to slightvariations in doping concentrations (e.g. in a planar configuration, thedepletion width of a Schottky-contact/depletion mode device scales with1/sqrt(concentration)).

3. In small-scale devices, larger variations are expected to become morelikely due to statistical reasons (e.g. at a doping concentration of10¹⁶ cm⁻³ one would expect on average one doping atom in a 50 nm sectionof a nanowire with a 50 nm diameter).

The invention is not limited to a structure containing only two devices,and a structure of the invention may contain three or more devices.

An elongate structure of the first device may be continuous with anelongate structure of the second device.

An elongate structure of the first device may be not continuous with anelongate structure of the second device whereby the first device isphysically separated from the second device.

The longest dimension of at least one elongate structure of the firstdevice may lie substantially on the same axis as the longest dimensionof at least one elongate structure of the second device. Preferably, forat least one elongate structure of the first device, an extension of itslongest dimension would lie within the perimeter of an elongatestructure of the second device (and vice versa).

The longest dimension of at least one elongate structure of the firstdevice may lie substantially on the same axis as the longest dimensionof at least one elongate structure of the second device, and the crosssectional geometry and the orientation of the cross-sectional geometryof these elongate structures may be substantially similar.

The first device and the second device may each include at least twoelongate structures, and the separation between a pair of elongatestructures in the first device may be substantially equal to theseparation between a corresponding pair of elongate structures in thesecond device.

The first device may emit light, in use, in a first wavelength range andthe second device may emit light, in use, in a second wavelength rangedifferent from the first wavelength range.

The first device may be a p-channel transistor whose conductance, inuse, is dominated by hole transport and the second device may be an-channel transistor whose conductance, in use, is dominated by electrontransport.

The elongate structures may be coplanar with one another.

The structure may comprise a group of first devices and a group ofsecond devices, wherein the material composition and materialcomposition profile of the elongate low-dimensional structures ofdevices of the first group are nominally the same for all elongatelow-dimensional structures of the first group; and wherein the materialcomposition and material composition profile of the elongatelow-dimensional structures of devices of the second group are nominallythe same for all elongate low-dimensional structures of the secondgroup.

In the current invention the low dimensional structures formed on theformation substrate may have a material composition, materialcomposition profile, cross-section geometry, cross-sectional dimensionsand/or orientation that varies along their largest dimension such that,if it is possible to form contacts in well defined locations along theirlargest dimension, two or more physically different devices may beobtained.

The term “contacts” includes electrical, optical, thermal and mechanicalcontacts, and any combination thereof.

The terms “material composition” and “material composition profile”include, but are not restricted to, doped semiconductors as well ashetero junctions.

In an embodiment of the current invention, encapsulated low dimensionalstructures are transferred to a receiver substrate such that it becomesor remains possible to form contacts anywhere along their largestdimension using at least one of the following techniques:

1. Additive (e.g. deposition, transfer)

-   -   Deposition methods include but are not restricted to direct or        indirect thermal evaporation, sputter deposition, chemical        vapour deposition, spin coating, atomic layer deposition, and        ink-jet printing.    -   Transfer methods include dry transfer methods such as        stamp-based transfers, and device bonding as well as wet        transfer methods where the transfer of the desired structures        occurs out of solution.

2. Subtractive (e.g. etching, sputtering, dissolving)

-   -   Etching includes wet-chemical etching and dry etching (e.g.        reactive ion etching, ion milling). Dry etching techniques may        be combined with sputtering techniques.

3. Selective (e.g. self assembly, chemical functionalisation, localheating, local exposure to particles, local exposure to mechanicalstress)

-   -   Local heating may occur due to a localised exposure to an energy        source (e.g. a focussed laser beam, selective exposure using a        mask) or due to the energy absorbing properties of the elongate        low dimensional structures or sections within the elongate low        dimensional structures.    -   Chemical functionalisation may utilise particular surface        properties of the elongate low dimensional structures being        defined by the material composition.    -   Local exposure of particles includes lithographic methods such        as photo-lithography and electron beam lithography but also        focussed ion beam and local exposure to x-rays. Local exposure        to mechanical stress includes imprint technologies.

The formation of contacts may be achieved by rotating high aspect ratiostructures such that their shortest dimension, denoted ‘w’ in FIGS. 1(c) and 2(a) extends perpendicular of the receiver substrate, henceeasing lateral structuring by application of lithographic methods.

The formation of contacts at well defined positions along each elongatelow-dimensional structure may remain possible if the alignment and theorientation of high aspect ratio structures is preserved during rotationand transfer to a different substrate (hence their positions remainknown).

High aspect ratio structures include elongate low-dimensional structuresas well as the composite structures described in UK patent applicationNo. 0620134.7.

In another embodiment of the current invention, the encapsulated lowdimensional structures are processed such that contacts are formed inplaces along their largest dimension, which are particularly suited toobtain devices performing different physical functions. The position ofthese places is defined by:

1. The material composition of these places along the elongate lowdimensional structures;

2. The material composition in between the places particularly suitedfor formation of contacts;

3. The physical extension of the material composition of these placesalong the elongate low dimensional structures;

4. The physical extension of the material composition in between theplaces particularly suited for formation of contacts; and

The material composition in between the places particularly suited forformation of contacts may have any symmetric or asymmetric compositionalprofile which is suited to obtain the desired device performance.

In a further embodiment of the current invention, the encapsulated lowdimensional structures may be processed such that they are separated toobtain multiple elongate low dimensional structures. The position wherethey are separated are defined by:

1. The material composition in between the places particularly suited toseparate these elongate low dimensional structures if selectivesubtractive techniques are applied.

2. The position of those parts of the elongate low-dimensionalstructures which are important to achieve the desired device performance(e.g. contacts and active device regions).

The separation of the elongate low-dimensional structures may beachieved by using a suitable subtractive method in combination with aselective method. Examples of these methods are given above.

Advantages of the current invention over the prior art are as follows.

1. Only one transfer process is required to obtain a well defined number“n” of physically different devices (n>1) in well defined locations onthe receiver substrate. Consequently, no alignment between differenttransfer steps is required. Furthermore, the final spacing between thephysically different devices is solely determined by the precision ofthe techniques used to fabricate the elongate low-dimensional structureson the formation substrate. These techniques are additive, henceallowing in most cases the fabrication of physically different segmentsalong these elongate low dimensional structures with greater precisionthan achievable with conventional lithographic techniques and devicetransfer techniques. As a result, two different devices may bemanufactured in close proximity to one another and a greater devicedensity can be obtained. However, other techniques to obtain physicallydifferent sections may be utilised before the structures are transferredto a receiver substrate.

2. Using the methods described in the prior art to fabricate “n”transferable and physically different groups of devices requires “n”separate fabrications—which are most likely done on “n” separatesubstrates if the previous point 1 is required. The current inventionallows the fabrication of elongate low dimensional structures only onceon one substrate. Hence, a large number of process steps to fabricatethese elongate low dimensional structures or any transferable structurecontaining one or more of these elongate low dimensional structures areperformed once only. This has the following advantages:

-   -   Fewer resources are required to fabricate these structures,        making the process more inexpensive, faster or both.    -   Each process step is associated with a certain yield. Therefore,        a reduction in process steps will improve the yield.

3. Often, the fabrication of the elongate low-dimensional structures isassociated with a certain yield and reproducibility. This may causevariations in the number of elongate low-dimensional structures andtheir cross-sections. One example where this is often the case nanowireswhich are created by catalysed growth, where minor variations in theinterface quality between the growth substrate and catalyst can impactthe catalytic properties and the wetting properties of the catalystwhich are thought to affect the nucleation yield and the nanowire crosssection. In some applications, however, it is important to havephysically different devices containing the same number of nanowireswith substantially similar cross sections in close proximity. CMOS logicgates are such an example where the number of nanowires times thecircumference of each nanowire determines the channel width. Here, theratio of the channel width of the pnp device and the channel width ofthe npn device belonging to the same CMOS logic gate needs to staywithin well controlled margins. Another example are light emittingdevices for colour mixing where the colour obtained is more importantthan the absolute brightness. If, for example, a pixel display isrealised with each pixel consisting of three LEDs, one emitting light inthe red, one in the green and one in the blue spectrum, it may be moreimportant to obtain reproducible colours than obtaining reproduciblebrightness. As the cross sectional area in each LED determines theoutput power, the ratio of the cross sections for the red, green andblue LEDs within each pixel needs to stay constant from pixel to pixel.Other applications may be in the fields of polarisation dependentphoto-detectors and polarised light sources.

The structure which will be obtained by the method described in thisinvention achieves these requirements (same number of wires per device,similar cross sections, close and well defined proximity of thedevices).

Additional objects, features, and strengths of the present inventionwill be made clear by the description below. Further, the advantages ofthe present invention will be evident from the following explanation inreference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described byway of illustrative example with reference to the accompanying Figuresin which:

FIG. 1 shows the formation of a group of low dimensional structures on asubstrate 3;

FIG. 2 shows the structures of FIG. 1 being processed further intophysically different devices after transfer to another substrate;

FIG. 3 illustrates further examples of structures obtainable by theinvention;

FIG. 4 illustrates a further example of a structure obtainable by thepresent invention;

FIG. 5 illustrates further structures obtainable by the presentinvention;

FIG. 6 shows the formation of a group of low dimensional structures on asubstrate containing two segments with different cross-sectionalgeometries and cross sectional areas;

FIG. 7 shows the formation of a group of low dimensional structures on asubstrate containing two segments with different cross sectional areas;

FIG. 8 illustrates that the formation of cross-sectional areas may alsoresult in different cross-sectional geometries;

FIG. 9 illustrates steps of a method according to a further embodimentof the invention.

FIG. 10 illustrates steps of a method according to a further embodimentof the invention.

FIG. 11 illustrates steps of a method according to a further embodimentof the invention.

FIG. 12 is a plan view of a device of the invention.

DESCRIPTION OF THE EMBODIMENTS

The invention will be described with reference to example in which theelongate structures are nanowires. The invention is not however limitedto nanowires and may be applied to other elongate structures.

FIGS. 1( a) to 1(c) and FIGS. 2( a) to 2(d) show principal steps of amethod according to one embodiment of the present invention.

Initially, a plurality of elongate low dimensional structures is formedover a substrate 3. The low dimensional structures may be formed onsubstrate 3 by an additive process, or they may be formed by subtractivemethods, such as lithography and etching. In this embodiment the lowdimensional structures 5 are nanowires, but the invention is not limitedto these. In this embodiment the elongate low dimensional structures areformed to be free-standing on the substrate 3, for example such that theelongate low dimensional structures are connected to the substrate 3only at one of their ends and extend generally away from the substrate.

One formation method suitable for use in the invention is a catalyticgrowth method. In such a growth method, a suitable catalyst 4 isinitially disposed on the surface of the formation substrate 3 at everylocation where it is desired to form an elongate low-dimensionalstructure 5 (FIG. 1( a)). The catalyst 4 may be, for example, a metalcatalyst. The catalyst may be deposited by, for example a combination ofsub-micron lithography/imprinting and lift-off, or by the deposition ofmetal colloids.

Next, as shown in FIG. 1( b), elongate low-dimensional structures areformed at each location where the catalyst 4 was deposited on thesurface of the substrate 3. Formation of elongate low-dimensionalstructures 5 does not occur at locations where the catalyst 4 is notpresent.

The low dimensional structures formed on the formation substratepreferably have a substantially unidirectional orientation. In FIG. 1(b) the elongate low-dimensional structures 5 are shown as oriented withtheir longitudinal axes generally perpendicular to the substrate 3.

The elongate low-dimensional structures may be formed by any suitabletechnique, for example by epitaxial vapour-liquid-solid or catalyst-freechemical vapour deposition or molecular beam epitaxy, or they may beformed by deposition of material in a porous sacrificial template. Theseare examples of “additive” formation processes, in which material is“added” so that a low-dimensional structure is formed at each locationwhere the catalyst 4 was deposited. As an alternative, a subtractiveformation process such as sub-micron lithography and etching may also beused. For example, silicon nanowires may be formed using an Au catalystin a (111) surface of a silicon formation substrate. The material of theelongate low-dimensional structures can be any suitable material suchas, for example, semiconductors, silicides, metal oxides, nitrides andany combination of the aforesaid materials. The elongate low-dimensionalstructures may be formed a single crystal low-dimensional structure.

The growth process thus far described may be generally as described inUK patent application No. 0620134.7.

In this embodiment, the material composition along the elongatelow-dimensional structures 5 is varied during growth of the elongatestructures such that the material composition changes along the lengthof the elongate structures. The composition of the elongatelow-dimensional structures material will be varied such that at leasttwo sections I and II having different material compositions and/ormaterial composition profiles from one another (ie at least twophysically different sections) are present in the elongate structures.This allows at least two physically different devices to be subsequentlyobtained from one tape-like composite structure like the one shown inFIG. 1. In the example illustrated in FIGS. 1 and 2, two physicallydifferent devices 1 and 2 may be fabricated out of one tape-likestructure. However, the invention is not limited to two physicallydifferent devices and also includes any number of devices larger thantwo.

In the example illustrated in FIG. 1, a plurality of sub-sections 1 a, 1a, 2 a, 2 a are defined along the elongate low-dimensional structuressuch that their physical properties are particularly suited for theformation of contacts (four such subsections are defined in FIG. 1, butthe invention is not limited to this number). For simplicity, twocontact regions are labelled 1 a and two contact regions are labelled 2a. If electrical contacts are to be formed in these regions 1 a and 2 a,it may be desired that these regions are distinguishable from otherregions by having a very low electrical resistivity. In the case ofelongate low-dimensional structures consisting of a semiconductingmaterial, this can be achieved by incorporating a sufficiently largeamount of doping atoms during the creation of these segments. In thisexample, the contact regions 1 a are separated by a first active deviceregion 1 b (that will ultimately form the active region of one device)while the contact regions 2 a are separated by a second active deviceregion 2 b (that will ultimately form the active region of anotherdevice). In this embodiment, the sub-sections comprising the activedevice region 1 b and the contact regions 1 a will consist predominantlyof a different material composition from one another. Similarconsiderations apply to contact regions 2 a and active device region 2b. However, more importantly, the material composition in sub-sections 1a differs from the material composition in sub-sections 2 a and/or thematerial composition in sub-sections 1 b differs from the materialcomposition in sub-sections 2 b. Consequently, the material compositionprofile in section I differs from the material composition profile ofsection II, rendering both sections to be physically different.

In the example given above of fabricating an n⁺ n⁻ n⁺ transistor as afirst device and an p⁺ n⁻ p⁺ transistor as a second device, the contactregions 1 a would differ from the contact regions 1 b, but the activedevice region 1 a need not differ from the active device region 1 b. Inanother example, for example the fabrication of light-emitting devices,the active device region 1 a would differ from the active device region1 b (to give different optical emission properties) but the contactregions 1 a need not differ from the contact regions 1 b. In furtherexamples, the contact regions 1 a may differ from the contact regions 1b and the active device region 1 a may differ from the active deviceregion 1 b.

The section I of an elongate structure containing an active deviceregion 1 b will be different from (eg, will have a different materialcomposition, a different material composition profile, a differentgeometry etc) from the section II of the elongate structure containingthe active device region 2 b. Subsequently, section I is processedfurther forming one device which is made up of sub-sections 1 a and 1 b,while section II is processed further forming the other device which ismade up of sub-sections 2 a and 2 b. In this example, the sections I andII are different in terms of doping concentration and doping profile,but this may also coincide with a change in diameter. For example,different gas compositions and possibly also temperatures used for thegrowth will change the surface tension of the metal catalyst required togrow nanowires. This will change the shape of the liquid catalyst, henceits contact area on the nanowire and could finally result in a differentnanowire diameter.

Next, the elongate low-dimensional structures 5 are encapsulated in amatrix 6 (FIG. 1( c)). This may be achieved by a conformal depositionprocess creating one or more layers of encapsulant material over all theexposed surfaces of the elongate low-dimensional structures and thesubstrate 3 to form the matrix 6, for example using a substantiallyisotropic deposition method such as chemical vapour deposition. It maybe advantageous if the matrix 6 is formed to a thickness sufficient tofill the spaces between adjacent elongate low-dimensional structures.

A substantially isotropic deposition method ensures that all elongatelow-dimensional structures are placed in the centre of the matrix, andthis is true for any fin-like structure formed on a common substrate.Furthermore, if the elongate low-dimensional structures belonging to onefin-like structures are fabricated such that they are positionedsubstantially within one common plane (e.g. all of the longest axes ofeach structure lie within the same plane). knowing the exact position ofthe elongate structures is advantageous for subsequent processing steps.It is also beneficial if the matrix material used is sufficiently rigidto avoid any substantial bending of the elongate low-dimensionalstructures. The use of a rigid or semi-rigid matrix and obtainingelongate structures positioned in the centre of the fin reduces thestrain inserted on the elongate structures due to forces applied to thefin. These forces may either occur in subsequent processing steps (e.g.transfer and bonding of the fin to another substrate) or in the finalproduct (e.g. flexible displays). UK patent application No. 0620134.7,discloses a method with which structures fulfilling these requirementscan be obtained.

Furthermore, the use of the method disclosed in co-pending UK patentapplication No. 0620134.7 to make the structures illustrated in thecorresponding figures in this invention makes it possible to harvest thebenefits which nanowires offer if the intended application does notrequire the ability to address individual nanowires but uses thecollective response of nanowires operated in parallel. This way,generally envisaged advantages such as improved gating due towrap-around gates or new phenomena exploiting the low-dimensionalgeometry can be utilised.

The material(s) used for the matrix 6 in this embodiment is/areconstrained to those which are compatible with the particular formationmethod. In a case where a chemical vapour deposition process is used,suitable materials include silica and degeneratively doped polysilicon.Atomic layer deposition may be applied to deposit materials with ahigher dielectric constant. Depending on the desired final structure,the matrix 6 may be, for example, light-transmissive, electricallyconductive, electrically insulating etc.

The elongate low-dimensional structures may be encapsulated so as toform one or more groups, as described in UK patent application No.0620134.7. FIG. 1( c) shows elongate structures encapsulated as a groupto form a “fin-type structure”. Additional details in how to fabricatethe fin-type structure shown in FIG. 1( c) can be found in UK patentapplication No. 0620134.7, the contents of which are hereby incorporatedby reference.

Next, the fin-type structure 1(c) is rotated. This rotation may be doneon the formation substrate 3, or after transfer to a different substrate7 yielding the configuration illustrated in FIG. 2( a).

Since the contact area between the fin-type structure shown in FIG. 1(c) and the substrate 3 is relatively low, it is much easier to removethis fin-type structure from the substrate 3 than it is to remove thecomposite structure of U.S. Pat. No. 7,091,120 from its formationsubstrate. It is also important to note that, if severalsufficiently-spaced fin-type structure are produced on one planarsubstrate surface according to the method illustrated in FIG. 1, and ifthese are rotated to arrive at the arrangement illustrated in FIG. 2(a), all elongate structures are coplanar.

The invention is not limited to fin-like structures which aretransferred to a different substrate, but may be also applied toindividual elongate structures which may or may not be encapsulated andit may also apply to non-transferred structures (e.g. where substrate 3and substrate 7 are the same substrate).

Alternatively, the nanowires may be fabricated on the substrate 7 butsuch that they are not initially oriented parallel to the substrate, andthen re-oriented so as to extend generally parallel to the substrate7—for example the nanowires may be fabricated with a flexible joint asdisclosed in co-pending UK patent application No. 0805846.3, thecontents of which are hereby incorporated by reference, so as tofacilitate re-orientation of the nano-wires so as to extend generallyparallel to the substrate 7.

The method of UK patent application. No. 0805846.3 comprises forming aflexible element in a low-dimensional structure, for example a nanowire,such that the flexible element has different elastic properties to abody portion of the low-dimensional structure.

In principle, the flexible element of UK patent application No.0805846.3 may be formed as the low dimensional structures are grown. Forexample, the flexible element may be realised by providing one or moreof the nanowires with a portion having a reduced cross-sectiondimension. When nanowires are grown using a metal catalyst, the surfacetension of the catalytic metal used during growth of nanowires affectsthe contact area between the catalyst and the already grown parts of thenanowires. This contact area determines the nanowire diameter. Varyingthe surface tension, for example by varying the temperature and/orgas-composition, will thus affect the diameter of the nanowires andallow the nanowires to be grown with a section having a reduceddiameter, compared to the diameter of the nanowires at other pointsalong their lengths. In general, however, it is expected that it will bemore convenient to form the flexible element after the elongatestructures have been grown.

Providing the flexible element of UK patent application No. 0805846.3may comprise providing, in a low-dimensional structure, a first portionthat has different elastic properties to a second portion, the firstportion being at a different axial position along the low-dimensionalstructure to the second portion. Providing the flexible element maycomprise reducing the stiffness of this portion, or forming the portionwith a lower stiffness, in comparison to the other sections—by eitherrealising a reduced second moment of area of the first portion or bychoosing a lower elastic modulus or both.

Forming the flexible element of UK patent application No. 0805846.3 maycomprise making a cross-sectional dimension of a first portion of alow-dimensional structure less than the corresponding cross-sectionaldimension of a second portion of the low-dimensional structure, wherebythe first portion of the low-dimensional structure comprises theflexible element. For example, in the case of a cylindricallow-dimensional structure, forming the flexible element may comprisemaking the diameter of the first portion less than the diameter of asecond portion. Reducing the diameter of a portion of the lowdimensional structure is a straightforward way of obtaining the flexibleelement, and the properties of the flexible element can be selected bychoice of appropriate values for the length and diameter of thereduced-diameter portion of the low dimensional structure. This methodis not however limited to reducing the diameter of the first portion, ieto making the first portion smaller in two dimensions, and it may alsocomprise thinning only one dimension of the first portion. This may beachieved by using a directional etch (eg, physical sputtering,exploiting etches or oxidation steps whose rates depend on the crystalorientation). When applied to a cylindrical low-dimensional structure,this would result in a first portion with a cross-section that isgenerally oval.

Alternatively, in the method of UK patent application No. 0805846.3 thelow-dimensional structures may be adhered to the substrate using alow-elastic modulus adhesive such that the adhesive acts as the flexibleelement.

In formal terms, what is required to form a flexible element in themethod of UK patent application No. 0805846.3 is forming or providing afirst portion of a low-dimensional structure with a cross-section thathas a lower second moment of area than the cross-section of the secondportion.

Where the method of UK patent application No. 0805846.3 is applied to agroup of low-dimensional structures, it may not be necessary to reducethe cross-sectional dimension of a portion of each low-dimensionalstructure in order to form the flexible element. Provided that asufficient number of the structures are provided with a portion that issufficiently flexible and strong so that the group of structures as awhole remains connected to the substrate during the re-orientationprocess it does not matter if others of the low-dimensional structuresshould fracture when the structures are re-oriented or if others of thelow-dimensional structures have inadvertently been over-thinned.

In this embodiment an important function of the matrix 6 is tosupport/lock the elongate low-dimensional structures 5 in a fixedposition relative to one another, so that the position, orientation andalignment of elongate low-dimensional structures 5 in a fin-typestructure, relative to other elongate low-dimensional structures in thatfin-type structure, are preserved during the removal of the fin-typestructure from the formation substrate 3 and its transfer to the targetsubstrate 7, and also to provide a handle by which the elongatelow-dimensional structures 5 can be simultaneously detached from theformation substrate 3 and transferred to the target substrate 7.

The term “fin-type” structure is used herein to denote a structure witha high aspect ratio, in which the shortest dimension of the encapsulant(denoted as ‘w’ in FIGS. 1( c) and 2(a)), extends parallel to a surfaceplane to which the structure is attached.

The elongate low-dimensional structures may be transferred to the targetsubstrate 7 such that the orientation of the elongate low-dimensionalstructures 5 relative to the target substrate 7 is different from theorientation of the elongate low-dimensional structures 5 relative to theformation substrate 3. For example, as shown in FIG. 2( a), the fin-typestructure may be deposited on the target substrate such that thelongitudinal axes of the elongate low-dimensional structures 5 extendgenerally parallel to the target substrate 7. The fin-type structurethus becomes a “tape-type structure” on the target substrate 7. The term“tape-type” structure is used herein to denote a structure with a highaspect ratio, in which the shortest dimension ‘w’ of the encapsulant(shown in FIG. 2( a)) extends perpendicular to a surface plane to whichthe structure is attached.

In addition to the particular variation of the material compositionalong the elongate low-dimensional structures 5, this invention differsfrom UK patent application No. 0620134.7 in the way the tape-typestructure is patterned and contacts are formed.

Once the fin-type structure has been transferred to the target substrate7, the matrix 6 can optionally be partially or completely removed toleave an array of partially or completely exposed elongatelow-dimensional structures 5 that can be subsequently processed into atleast two physically different devices 1 and 2 along the largestdimension of the elongate low-dimensional structures 5.

In the example illustrated in FIG. 2( b) the matrix 5 is partiallyremoved to expose the contact regions la and contact regions 2 a.Portions of the matrix that remain form contacts 1 c and 2 c tosub-sections of the elongate structures, in this example to thesub-sections 1 b,2 b that will form the active regions of the devices.The exposed contact regions 1 a and 2 a are subsequently encapsulatedwith a suitable contact material 1 d, 1 e, 2 d, and 2 e, as shown inFIG. 2( c). If electrical contacts are desired, materials with a highelectrical conductivity from the group of metals or silicides may bedesirable. Finally, some or all of the material of the elongatelow-dimensional structures may be removed between contact 1 d andcontact 2 e by using subtractive techniques to physically separatedevice 1 from device 2 (FIG. 2( d)).

In this way, a circuit structure that includes two physically differentthree-terminal devices 1 and 2 may be obtained on substrate 7. In thisexample, each device contains the same number of elongate structures.The devices are in close proximity to one another and each pair ofnanowires which previously formed, before its separation into twonanowires, one nanowire will be well aligned to each other (eg, thelongest dimension of one nanowire of the pair lies substantially on thesame axis as the longest dimension of the other nanowire of the pair)while, in the case of a circular cross section, the ratio of the radiiof both nanowires formerly belonging to one nanowire will differ lessfrom nanowire to nanowire than the absolute values of the radii.

In this embodiment, the material composition, material compositionprofile, cross-section geometry, orientation of cross-sections,cross-sectional dimensions and orientation of the elongatelow-dimensional structures of the first device are nominally the samefor all elongate low-dimensional structures of the first device 1.Similarly, the material composition, material composition profile,cross-section geometry, orientation of cross-sections, cross-sectionaldimensions and orientation of the elongate low-dimensional structures ofthe second device are nominally the same for all elongatelow-dimensional structures of the second device 2. However, at least oneof the material composition, material composition profile, cross-sectiongeometry, cross-sectional dimensions and orientation of the elongatelow-dimensional structures of the first device 1 differs from thematerial composition, material composition profile, cross-sectiongeometry, cross-sectional dimensions and orientation of the elongatelow-dimensional structures of the second device 2, so that the firstdevice is physically different from the second device.

In connection with the term “orientation of cross-sections”, nanowiresand other elongate structures are typically grown on a crystallinesubstrate and the crystal orientation of the substrate will determinethe cross-sectional geometry of the elongate structure and (assumingthat the cross-section is not circular) its orientation with respect tothe longest axis of the structure. Since the structures are all formedon the same substrate they should, after being transferred to adifferent substrate, still have the same orientation with respect toeach other assuming that, for example, the tape-approach of co-pendingUK patent application No. 0620134.7 is used to “lock them in”. Moreover,the orientation of the cross-sections may now be characterised withrespect to the different substrate. This would not be the case, however,if the elongate structures were dispersed in solution and rearranged, asthey might then have different orientation or cross-section as theymight be rotated along their longest axis. (It should be noted that thegeometry of the cross section may change owing to subsequent processing,for example as described below with reference to FIG. 8.)

Provided that the elongate structures shown in FIG. 2( a) weretransferred to the substrate 7 in such a way to preserve the orientationof their cross-sections, as originally grown on the substrate 3, allelongate structures in the first device should have the same orientationof cross-section as one another, and all elongate structures in thesecond device should have the same orientation of cross-section as oneanother. However, the elongate structures in the second device need nothave the same orientation of cross-section as the elongate structures inthe first device, for example if the elongate structures were grown suchthat section I of the structures had a different orientation ofcross-section (or even a different cross-section) to section II of thestructures.

The typical spacing between the devices 1,2 depends on the method usedto define the devices. If, for example, the devices do not need to beelectrically insulated from each other by removing sections of thenanowires, the spacing is defined by the control available from thegrowth tool. CVD and MBE systems can switch from one material to thenext within a few, sometimes even one, atomic layer. Also, if a specialsection can be etched much faster than another one because it is moreselective to a particular etch, the technique used to grow the nanowiresmay ultimately define the spacing.

If, on the other hand the nanowires need to be broken up by removingsections in order to define different, electrically insulated devices,without the possibility of defining a suitable different sacrificialmaterial, the spacing between devices will depend on:

1. The resolution of the lithography used to define the gap between thedevices

2. The variation of the expected position of each tape. This woulddepend on the quality of the transfer of the tape if the tapes are to betransferred to a target substrate: e.g. most adhesives require highertemperatures allowing to cross-link them. Furthermore, they are fairlycompliant at high temperatures. As a result, different thermal expansioncoefficients between the fabrication substrate and the target substrateand the softness/flow of the adhesive may result the tapes to end up indifferent positions. These variations need to be taken care of if(optical) lithography is used for separating the nanowires intodifferent segments—e.g. by choosing a larger gap. However, thissituation would be worse if two transfers which need to be aligned withrespect to each other are required because now the variations mentionedabove may be less uniform—be different for the first transfer comparedto the second transfer, while in addition the second transfer needs tobe aligned to the first transfer.

The spacing between two devices that may be obtained will depend on theresolution of the lithography and the uniformity of the transfer, butnot in addition to the alignment accuracy of the transfer (the alignmentaccuracy of the transfer will be probably worse than the precision oflithography and uniformity of the transfer due to the reasons mentionedabove). The present invention can therefore reliably produce smallerdevice spacings than can prior methods.

One particular example where this outcome is desirable would be therealisation of Si-based CMOS circuits on glass substrates. If substrate7 is glass, and if the elongate low-dimensional structures 5 consist ofsilicon with contact regions 1 a being highly p-doped, contact regions 2a being highly n-doped, active device region 1 b being n-doped andactive device region 2 b being p-doped one would obtain npn and pnptransistors. Contact 1 c and contact 2 c would be the gates, contact 1 dand contact 2 d the drains, while contact 1 e and contact 2 e would bethe sources.

In another embodiment it might be desirable to combine one or more twoterminal devices such as sensors or light emitting devices with one ormore three terminal devices such as transistors. In this case, theexample in FIG. 3( a) illustrates the corresponding outcome where oneterminal (in this particular case contact 1 c as it is labelled in FIG.2( b)) has been removed for example, during removal of the matrix 5.This produces a structure containing one two-terminal device 1 and onethree-terminal device 2. The resulting two-terminal device could, forexample, be a sensor where the active device region is now exposed tothe environment. If a light emitting diode is to be integrated with atransistor it might be advantageous to use a transparent matrix 6,allowing the matrix to be left around the active region of the lightemitting diode which may have a positive impact on the deviceperformance and device lifetime. However, as most transparent materialsare poor conductors, it might be necessary to replace the remainingmatrix (labelled as 2 c in FIG. 2( b)) on device 2 with a metal contact2 c to form a suitable transistor (e.g. a MES-FET).

In yet another embodiment two or more physically different two-terminaldevices might be desired for example to integrate light emitting deviceseach emitting at a different wavelength, on glass substrate. In thiscase, contacts 1 c and 2 c as labelled in FIG. 2( b) might be removedunless they consist of a suitable transparent and non-conductivematerial. Such a material could be silicon dioxide or silicon nitride.For display applications it might be particularly attractive tointegrate more than 2 devices emitting light at more than 2 differentwavelengths in the red, green and blue spectrum. FIG. 3( b) shows astructure containing two two-terminal devices.

In another embodiment of this invention, no segments of the elongatelow-dimensional structures 5 are removed because their physicalseparation may not be desired. FIG. 4 illustrates such an example wherecontacts 2 e and 1 d of FIG. 3( a) are not fabricated [FIG. 4], yieldingone three-terminal device 2 and one one-terminal device 1—contact 1 d iscommon to both devices. Device 2 may be a transistor switching device 1,which may be a light-emitting device or a sensor.

In yet another embodiment of this invention, the concept described isextended to groups of devices, each group containing physicallyidentical devices while devices belonging to different groups arephysically different. In one particular example, devices belonging tothe same group may be physically adjacent to each other. This isillustrated in FIG. 5, where devices 8 a and 8 b are physicallyidentical and belong to group 8 and devices 9 a and 9 b are physicallyidentical and belong to group 9. For clarity it is noted that thisinvention is not restricted to two devices within two groups but to anynumber of devices within each group and any number of groups.Furthermore, the spatial arrangement of the devices may be realised inany conceivable order along the largest dimension of the elongatelow-dimensional structures. All or some of the devices may be physicallyseparated from each other by removing segments of the elongatelow-dimensional structures. FIG. 5( b) shows all devices beingphysically separated from each other, whereas FIG. 5( a) shows theelongate structures continuous through the devices.

In the figures, the elongate low-dimensional structures are shown ashaving a uniform cross-section along their length. In general, however,the one-dimensional geometrical parameters describing the cross sectionsof the elongate low-dimensional structures contained within one device 1may differ by a factor “m” from the corresponding one dimensionalgeometrical parameters describing the cross-sections of thecorresponding elongate low dimensional structures of a second device 2.The value of “m” does not change significantly between equivalent pairsof devices found on the same receiver substrate.

In the methods of the invention the obtained circuit structure mayundergo further processing steps, for example to deposit an insulatinglayer over the devices and/or to provide conductive leads to thecontacts. The substrate 7 may be the final substrate, or it may be anintermediate substrate and the structure may undergo a further transferto another substrate.

In the previous sections the invention only refers to the implementationof physically different devices by different material compositions andmaterial composition profiles. Next, other means to realise physicallydifferent sections are discussed.

The formation of contacts which includes lithography, removing parts ofthe matrix and depositing a conductive layer is easiest if the matrix 6is as thin as possible and the elongate low-dimensional structures arelying in a common plane parallel to the substrate. This makes thestructures obtainable by the method disclosed in co-pending UK patentapplication No. 0620134.7 particularly suited.

FIGS. 6 and 7 illustrate how sections along elongate low dimensionalstructures with differing cross sectional dimensions and geometriescould be realised.

In one example [FIG. 6], the catalyst regions 4 may be used as an etchmask to form nanopillars 10 in substrate 3 by using an anisotropic etch[FIG. 6 a-6 b]. Subsequently the substrate is heated to the growthtemperature where the catalyst may become liquid. As a result, theinitially flat and disc-shaped catalyst regions will each form a dropletwhich will have a smaller contact area with the nanopillars than when ithad a disc-shaped geometry (not shown). The diameter of the subsequentlygrown nanowires 11 is determined by this contact area and as a resultthe nanowires 11 will have a smaller cross section than the nanopillars10, as shown in FIG. 6 c. Conversely, If the catalyst regions 4 have agreater height than shown in FIG. 6 a, this would increase the contactarea between a catalyst region and the underlying nanopillar when thecatalyst region is heated to its liquid state and it would be possibleto finally obtain nanowires 11 with a larger cross sectional area thanthe one of the nanopillar 10.

FIG. 7 highlights another possibility to modify different segments alongelongate low dimensional structures after growth of the low dimensionalstructures thereby to produce physically different segments, by exposingthese sections by partially removing the matrix 6 [FIG. 7 a-7 b] andsubsequently modifying the exposed sections 5 b while the sections 5 aremain untouched [FIG. 7 c]. FIG. 7 illustrates thinning as one exampleto modify a well defined segment which can be done by any suitablesubtractive method or, as in the case of silicon structures, by thermaloxidation and subsequent removal of the oxide. However, other method ofmodification may include modifying the material composition (e.g. byimplantation or diffusion). The low dimensional structures may have beeninitially grown as free-standing on substrate 7, encapsulated, androtated to be oriented parallel to the substrate 7. This may involvesevering the connection between the free-standing low dimensionalstructures and the substrate 7, or alternatively, the low dimensionalstructures may be fabricated on the substrate 7 with a flexible joint asdisclosed in co-pending UK patent application No. 0805846.3, so as tofacilitate re-orientation of the low dimensional structures so as toextend generally parallel to the substrate 7. Alternatively, the lowdimensional structures may have been initially grown as free-standing ona formation substrate and subsequently transferred to substrate 7.

It should be noted that the method of FIG. 7 results in the first andsecond physically different sections being defined in the elongatestructures after the elongate structures have been fabricated, whereasthe methods of FIG. 1 or FIG. 6, for example, result in the first andsecond physically different sections being defined in the elongatestructures as the elongate structures are grown on the first substrate3. That is to say, to reach the stage shown in FIG. 7( a), the elongatestructures 5 are grown on a donor substrate, encapsulated in a matrix 6,and transferred to the substrate 7.

The method of FIG. 7 does not require that first and second physicallydifferent sections are defined in the elongate structures when theelongate structures are grown.

In principle, it would be possible to apply the method of FIG. 7 toelongate structures in which first and second physically differentsections had been defined at the growth stage, to define furthersections in the elongate structures. It is important to note onepeculiarity to epitaxial growth of nanowires using a liquid catalyst: Itturns out that it is energetically favourable that the nanowire exhibitsside-facets which coincide with crystal planes. For example, siliconnanowires grown off a [111] Si surface will generally have a hexagonalcross section with three major side facets and three minor side facets,all corresponding to {112} orientations of the substrate (dashed line inFIG. 8 a). Thermal oxidation is not only dependent on crystalorientation and doping concentration, but it also depends on the stresswithin the already created oxide layers which, in the case of siliconoxidation, influences the diffusion of oxygen to the oxide-siliconinterface. As a result a concave surface oxidises faster than a convexsurface at reasonably low temperatures (typically below 900 C). Thismeans, that oxidation at minor facets occurs at a much slower rate thanat major facets which will change the cross sectional geometry from ahexagonal to a more triangular shape [solid line in FIG. 8 a]. For thesame reason, the side facets of the triangular cross-section are oftencurved [solid line in FIG. 8 b].

FIG. 9 highlights another possibility to modify different segments alongelongate low dimensional structures which were formed as free-standingelongate low dimensional structures. As with the method of FIG. 7, theelongate low dimensional structures are modified after the structureshave been fabricated. In the method of FIG. 9 this is done by maskingthose segments which should remain unaltered. FIG. 9 shows the methodapplied to elongate low dimensional structures that extend away from,for example generally perpendicular to, a substrate but the method maybe applied generally to free-standing elongate low dimensionalstructures.

Initially, all free-standing elongate structures are immersed in anothermaterial 12, say a polymer, which can be for example deposited by spincoating or ink jet printing. Subsequently, this material is partiallyremoved applying a suitable etch (e.g. in the case of a polymer anoxygen plasma may be used) [FIG. 9( b)]. Next, the exposed parts of theelongate structures may be modified, e.g. they may be thinned as shownin FIG. 9 c (or alternatively may for example have their materialcomposition changed by implantation or diffusion). Afterwards, themasking material may be removed [FIG. 9 d] and the elongate structuresmay be processed as discussed in previous embodiments. For example,after the low dimensional structures have been modified as describedwith reference to FIGS. 9( a)-9(d), the connection between the lowdimensional structures and the substrate may be severed, for example toallow the low dimensional structures to be re-oriented to be parallel tothe substrate or to be transferred to another substrate. Alternatively,the low dimensional structures may be fabricated on the substrate 7 witha flexible joint as disclosed in co-pending UK patent application No.0805846.3, so as to facilitate re-orientation of the low dimensionalstructures so as to extend generally parallel to the substrate.

Thus, in the method of FIG. 9 the low-dimensional structures aremodified after they have been fabricated, as in the method of FIG. 7.Unlike the method of FIG. 7, however, in the method of FIG. 9 thelow-dimensional structures are modified while they are still on thesubstrate on which they are fabricated.

The density of elongate low dimensional structures can be varied fromone section to another section by exploiting different growth rates(e.g. nanowires of different sizes will grow with different growthrates). As a result, the faster growing nanowires could be used to formone device while all nanowires could be used to form the other device,

The orientation of the nanowires grown by the method of FIG. 6 maydiffer from the orientation of the nanopillars if the crystalorientation along the longest dimension of the nanopillar differs fromthe crystal orientation along the nanowires grow. This change inorientation could be facilitated to obtain a higher nanowire density.

Furthermore, kinks can be introduced during nanowire growth by changingthe gas flows (which is often the case if the material composition isabout to change.) If these kinks occur for all nanowires in the samedirection and if this direction extends partially along the directiondescribed by the row of catalysts 4 in FIG. 1 a, once again the densityof the elongate low dimensional structures is increased.

FIG. 10 illustrates another embodiment of this invention where elongatestructures are formed between two structures 13 a and 13 b which areformed on a substrate 13 [FIG. 10( a)].

Substrate 13 may be an insulator such as silicon di-oxide, thestructures 13 a and 13 b may be semi-conducting or conducting (e.g. theymay consist of silicon or a silicide).

The elongate structures contain physically different sections 14 a and14 b. (Although the sections 14 a, 14 b are each shaded uniformly inFIG. 10 they may, as explained above, contain two or more sub-sectionsof differing properties.)

If three terminal devices such as transistors are desired, the structuremay be encapsulated with a thin dielectric layer (not shown) andcontacts 15 a and 15 b may be formed which remain isolated from theelongate structures by the dielectric layer [FIG. 10( b)]. Finally, aelectric contact 16 is formed which electrically contacts the elongatestructures. The last step requires the removal of the dielectric layerwhere this contact is to be formed.

Thus, in this embodiment one device comprises the sections 14 a and thecontacts 13 a, 15 a and 16, and the second device comprises the sections14 b and the contacts 16, 15 b and 13 b.

Previous embodiments refer to the fabrication of devices formed usingestablished planar technologies. It was pointed out that this inventionis particularly suited if these devices are to be realised not on thesubstrate on which the elongate structures are formed but on a differentsubstrate. Also, if all elongate structures are to be aligned inparallel to each other the formation of elongate structures protrudingvertically off a substrate surface is advantageous. However, it ischallenging to process vertical elongate structures into devices whichis why the rotation of these structures such that they lie in a planeextending parallel to the substrate surface is often desirable.

The challenge of applying this invention to a vertical structurecontaining physically different sections 14 a and 14 b such as the oneshown in FIG. 11( a) without rotating it is illustrated by FIG. 11( b):The bottom contact 13 b can coincide with the substrate from which theelongate structures protrude and the top contact 13 a may be implementedby encapsulating the structure with a dielectric 18, and polishing thislayer to expose the top end of the elongate structures before depositingthe contact material. However, the technological challenge lies infabricating electrical contacts 15 a, 15 b and 16 such that they fulfilthe same role as the contacts 15 a, 15 b and 16 shown in FIG. 10( b).This would require contacts 15 a and 15 b to be separated from theelongate structures by an insulating dielectric layer 17 while creatingcontact 16 requires a discontinuous dielectric layer with an openingwhere the electrical contact is to be formed.

If the structure shown in FIG. 11( b) can be realised, an inverter—thebasic building block of CMOS circuits—could be formed: the devicecontaining section 14 a being a p-channel transistor, the devicecontaining 14 b being an n-channel transistor, 15 a and 15 b being onecommon input, and 16 being the output of the logic gate while anappropriate potential difference is applied to contacts 13 a and 13 b.

FIG. 12 is a schematic plan view of a pair of devices fabricated by amethod of the invention. Each device comprises a plurality of nanowires(or other elongate structures) 19 a-24 a, 19 b-24 b, partiallyencapsulated in a matrix denoted schematically as 25 a,25 b. The deviceshave been manufactured by a method in which the nanowires are separatedin order to define the two separate devices, so that nanowire 19 a wasoriginally continuous with nanowire 19 b, and so on.

Both devices 18 a,18 b contain nanowires which lie on common axes (shownas dashed lines), which means that they are extremely well aligned toeach other (possibly much better aligned than is achievable bytransferring first one device and then the other device onto a commonsubstrate). Also, where the pitch between adjacent nanowires, forexample the pitch between nanowire 19 a and nanowire 19 b showsvariations (e.g. where nanowires have been removed), and/or the nanowirediameter show variations, these variations are substantially similar inboth devices. (They may not be identical from one device to another, asnanowires may bend before they are locked in by the matrix. Also, thedegree of variations in diameter might be size dependent in the sensethat thicker nanowires exhibit, in relation to their size, largerdiameter fluctuations as smaller nanowires do, but the overall signatureis expected to be substantially similar.) For example, the separationbetween nanowire 21 a and nanowire 20 a in device 18 a is substantiallysimilar to the separation between nanowire 21 b and nanowire 20 b indevice 18 b. Similarly, the ratio between the diameter of nanowire 22 aand the diameter of nanowire 21 a in device 18 a is substantiallysimilar to the ratio between the diameter of nanowire 22 b and thediameter of nanowire 21 b in device 18 b. In general terms, the crosssectional geometry and the orientation of the cross-sectional geometryof a nanowire (or other elongate structures) in device 18 a aresubstantially similar to the cross sectional geometry and theorientation of the cross-sectional geometry of a corresponding nanowire(or other elongate structures) in device 18 b.

Additionally, this method will have some advantages if devices need tobe “matched”, because variations from the desired outcome will affectboth devices in a similar way. This could be valuable to, for example,match p-channel with n-channel transistors (where the nanowire diameterdetermines the channel width) or in light-emitting devices where coloursneed to be mixed (red green and blue LEDs may have an overall poweroutput which is hard to predict, but the colour should not vary muchfrom one RGB-LED set to the next one—just the intensity).

The present invention is not limited to the description of theembodiments above, but may be altered by a skilled person within thescope of the claims. An embodiment based on a proper combination oftechnical means disclosed in different embodiments is encompassed in thetechnical scope of the present invention.

The embodiments and concrete examples of implementation discussed in theforegoing detailed explanation serve solely to illustrate the technicaldetails of the present invention, which should not be narrowlyinterpreted within the limits of such embodiments and concrete examples,but rather may be applied in many variations within the spirit of thepresent invention, provided such variations do not exceed the scope ofthe patent claims set forth below.

1.-24. (canceled)
 25. A circuit structure comprising: first and seconddevices disposed or formed over a common substrate, each devicecontaining elongate low-dimensional structures; wherein the first deviceis in close proximity to the second device; wherein at least one of amaterial composition, material composition profile, cross-sectiongeometry, cross-sectional dimensions and orientation of the elongatelow-dimensional structures of the first device differs from a materialcomposition, material composition profile, cross-section geometry,cross-sectional dimensions and orientation of the elongatelow-dimensional structures of the second device, whereby the firstdevice is physically different from the second device; wherein thematerial composition, material composition profile, cross-sectiongeometry, orientation of cross-sections, cross-sectional dimensions andorientation of the elongate low-dimensional structures of the firstdevice are nominally the same for all elongate low-dimensionalstructures of the first device; and wherein the material composition,material composition profile, cross-section geometry, orientation ofcross-sections, cross-sectional dimensions and orientation of theelongate low-dimensional structures of the second device are nominallythe same for all elongate low-dimensional structures of the seconddevice.
 26. A structure as claimed in claim 25 wherein an elongatestructure of the first device is continuous with an elongate structureof the second device.
 27. A structure as claimed in claim 25 wherein anelongate structure of the first device is not continuous with anelongate structure of the second device whereby the first device isphysically separated from the second device.
 28. A structure as claimedin claim 25 wherein a longest dimension of at least one elongatestructure of the first device lies substantially on a common axis with alongest dimension of at least one elongate structure of the seconddevice.
 29. A structure as claimed in claim 25 wherein a longestdimension of at least one elongate structure of the first device liessubstantially on a common axis with a longest dimension of at least oneelongate structure of the second device, and wherein the cross sectionalgeometry and the orientation of the cross-sectional geometry of theseelongate structures are substantially similar.
 30. A structure asclaimed in claim 28 wherein the first device and the second device eachinclude at least two elongate structures; and wherein a separationbetween a pair of elongate structures in the first device issubstantially equal to a separation between a corresponding pair ofelongate structures in the second device.
 31. A structure as claimed inclaim 25 wherein the first device emits light, in use, in a firstwavelength range and the second device emits light, in use, in a secondwavelength range different from the first wavelength range.
 32. Astructure as claimed in claim 25 wherein the first device is a p-channeltransistor whose conductance, in use, is dominated by hole transport andthe second device is a n-channel transistor whose conductance, in use,is dominated by electron transport.
 33. A structure as claimed in claim25 and comprising a group of first devices and a group of seconddevices, wherein the material composition and material compositionprofile of the elongate low-dimensional structures of devices of thefirst group are nominally the same for all elongate low-dimensionalstructures of the first group; and wherein the material composition andmaterial composition profile of the elongate low-dimensional structuresof devices of the second group are nominally the same for all elongatelow-dimensional structures of the second group.
 34. A structure asclaimed in claim 25 where the elongate structures are coplanar with oneanother.
 35. A circuit structure as claimed in claim 24 wherein theelongate structures are coplanar with one another.